PRT tries to combine the best aspects of private automobiles and public transportation. It uses car-sized pods that each carry just a few people at a time. Passengers arrive at a station and, rather than wait for a bus or train headed their way, they hail a pod-car. The pod-car then carries them to their destination via an elevated monorail track. Theoretically the PRT pods do this without stopping at intermediate stations.

PRT advocates say it provides point-to-point transit, meaning it goes everywhere a car can go, since, theoretically, a city could build elevated lanes atop any street.

It’s a compelling theory, which is why it’s been kicking around the periphery of the transit planning industry since at least the early 1970s.

Unfortunately it doesn’t work very well. It turns out that combining the convenience of personal cars with the efficiency of mass transit results in something that’s neither convenient nor efficient.

PRT is less convenient than cars

The basic idea of PRT is to build an elevated lane for special pod-cars that can only drive on that lane. The pod-cars could run on wheels or tracks, but the key characteristic is a small vehicle on its own elevated lane.

Meanwhile, the great thing about personal automobiles is that you can drive one anywhere there’s a road. PRT and transit both compromise that convenience by only offering service along routes, or PRT tracks.

In theory, PRT compensates for that inherent inconvenience by offering a dedicated elevated lane that can whiz pod-cars by faster than shared road lanes. But dedicated, elevated lanes are inherently expensive to build. Much more expensive than a normal surface street.

That means that unless we are willing to build an expensive duplicate infrastructure of new elevated lanes over every street in Arlington, by definition PRT cannot go everywhere roads can go. In turn, that means we have to prioritize the most important streets and design routes that carry people along only the most important paths.

In other words, we have to build a transit network.

And since elevated tracks are expensive, we can only build PRT on the most important, high-ridership corridors. Which brings up another problem:

It’s all about transit capacity

What happens when the new pod-car lane fills up?

PRT advocates like to show pretty renderings of little pod-cars whizzing over highway traffic without a care in the world. Those pictures rely on an assumption of low ridership. If more people rode the pod-cars, there would have to be either more pod-cars (leading to pod-car traffic jams), or much bigger pod-cars (ie buses for the pod-car lane).

PRT doesn’t change the basic math of congestion. One commuter takes up the same amount of physical space in a PRT pod-car as he or she would take up in a personal car, bus, or train. So by definition, PRT’s capacity is lower than transit, simply because PRT uses small vehicles.
And since we can only afford to build PRT in corridors with heavy transit demand, that means PRT’s low capacity is a real problem.
Theoretically PRT compensates for its low vehicle capacity in two ways, compared to buses: By running more pod-cars more often, and by giving its small vehicles their own elevated lane, which moves faster than buses on surface streets.
But on Columbia Pike buses already come extremely frequently at peak times. At that ridership level, smaller vehicles that come more often are not practical. The number of passengers on the line simply requires bigger vehicles.
Meanwhile, elevated lanes are great. But transit can have its own lane too. And transit on a dedicated elevated track will always have higher capacity than PRT.
So if Arlington is going to go to the trouble and expense of building a new elevated track atop Columbia Pike, why settle for low-capacity PRT? At that point, go ahead and make it elevated light rail. Columbia Pike is already long and straight and perfectly suited for that kind of high capacity transit in the first place.
Also, a lot of transit users on Columbia Pike are transferring to or from Metro. One Metro train unloading at the Pentagon could easily have hundreds of people waiting to board a PRT pod, which would lead to huge delays for those passengers and the system overall.
This is the inherent problem of PRT, and is why there are so few examples of it in the world: We can only afford to build elevated tracks on high-demand corridors, but on high-demand corridors, we need the capacity of transit.
We already know what the good solutions are
Of course, PRT is impractical since elevated lanes are expensive. But it’s interesting to note that there is already a transit system on Columbia Pike today that does sort of accomplish the same goals as PRT: Capital Bikeshare.

Capital Bikeshare stations in the vicinity of Columbia Pike. Image from Capital Bikeshare.

Of course, Capital Bikeshare doesn’t run automated pod-cars along dedicated elevated lanes. Instead, it runs manual-powered bikes along surface bike lanes. But in many ways, it’s a low-tech and low-cost PRT system.
And Capital Bikeshare is great. We should have more of it. But it hasn’t eliminated the need for transit on Columbia Pike.
Kudos for thinking outside the box, but let’s move on
It’s been nearly a year since the Arlington County Board canceled the Columbia Pike streetcar. Arlington residents are justifiably upset that there’s still no solid Plan B. Given the county government’s slow progress, residents deserve credit for thinking outside the box, and being open to new ideas.
Sometimes new ideas even prove worthwhile. The idea of a Georgetown gondola elicited eye-rolls at first, but people seem to be warming to it as Georgetown’s unique set of issues come into focus.
Even CaBi was a bit of a risk, but the program has worked because it fills a gap in demand for transportation around town for certain types of trips. But the issues on Columbia Pike are not the same issues that PRT or CaBi can solve.
Just being an out of the box idea doesn’t automatically make something smart. Often, such ideas have failed to take hold specifically because they’re impractical. Moreover, proposals like this can muddle conversations that have already been plagued with confusion and misinformation that led to the demise of the streetcar program in the first place.
That’s the case with PRT.

Canaan Merchant was born and raised in Powhatan, Virginia and attended George Mason University where he studied English. He became interested in urban design and transportation issues when listening to a presentation by Jeff Speck while attending GMU. He lives in Reston.